Time-delay circuit



Dec., 4, E951 I F, K, LOW 2577?@7 TIME DELAY CIRCUIT Filed Sept. 9, 1947 C GIS/DE NSE I? L opt-@A TE UL T/M TE 2 CL/.QREN T RELA Y WIND/IVG 57E/ImfsTATE VOLTAGE L1 VOLTAGE BETWEEN POINT P ANU @ROL/N0 A 7` TOR/VE V Patented Dec. '4, 1951 TIME-DELAY CIRCUIT Frank K. Low, Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 9, 1947, Serial No. 773,030 2 Claims. (Cl. 175-320) This invention relates to timing apparatus and more particularly to circuits for delaying the energization of a current-responsive device and thereby, in the case of a relay, delaying the attraction of its armature fora certain period of time after the circuit is established.

The usual means employed for delaying the operation of a relay involves a short-circuited winding or copper slug on the relay core or a special construction of the relay. Another means sometimes used comprises a gas lled break-down tube or vacuum tube in combination with a con denser-resistance combination which causes the relay to operate when the condenser becomes charged to a, given voltage by current cwing' through the resistance. .arrangements ci the rst type give more or less variable operation and those of the second type are rather expensive. An object oi this invention is to provide a simplined and inexpensive means for operating a relay after a definite and controllable interval, where by the relay may be oi standard and simple confstruction. Another object of this invention is to provide a means for operating a relay after a controllable interval, whereby variations in the relay operate time are a relatively small part ci the over-all time thereby permitting precise control over the time-delay interval. .A further object of this invention is to provide a means for accelerating the movement of the relay armature whereby increased pressure for the contacts may be obtained. Other objects and advantages of the invention are within the scope of the invention as will appear from the appended claims.

A complete understanding of the invention will be derived from the detailed description that iollows taken in conjunction with the appended drawings, wherein:

Fig. 1 is the basic circuit diagram of the invention;

Fig. 2 is a graph showing in qualitative manner typical voltage-time relations for the apparatus;

Fig. 3 is a graph showing in qualitative manner typical current-time relations for the apparatus;

Fig. 4 is a modification of the basic circuit permitting the use of a single source of potential; and

Fig. 5 is a modification of the basic circuit whereby increased pressure for the relay contacts may be obtained.

The basic circuit of the invention as shown in Fig. l comprises a unidirectional device such as unilateral cell I5 connected to two sources of potential l2 and I3, and the positive terminals of these sources of potential are grounded. The potential of source I3 is substantially less than that of source I2. One side of the unilateral cell I5 is connected to source of potential I2 through timing resistor R and bleeder resistor il. Condenser C is connected across resistors R and il. rthe other side of the unilateral cell lisconnected to source of potential i3 through the winding ci relay i Contacts l are connected to the junction between resistors R and i l and to ground.

kUnilateral oeil l5 will conduct current only when the lett-hand terminal of the cell, as shown in the drawings, is of positive polarity with respect to the right-hand terminal of the cell. When switch t@ is open no current is supplied to the circuit by either battery, and, since there is no potential drop across the resistors, the magnitude of the potential across cell l equals the diierence in potential between batteries i2 and it. Since the potential across battery i2 is larger than the potential across battery t3 and since the negative 'terminal of each battery is connected to cell i5, it follows that the left-hand terminal or cell i5 is of negative polarity with respect to the right-hand terminal oi the cell and the cell is in a non-conducting condition. Thus, neglecting leakage currents, no current flows in the circuit and the biasing potential across cell l5 is equal to the difference in potential between batteries I2 and i3. While the contacts lll are open the potential across timing condenser C is normally zero since there is no current flowing through resistors II and l2. When contacts I0 are closed current flows from the grounded pole of battery l2 through contacts l0 and resistor II to the negative pole of battery i2. The potential drop across resistor Il which is caused by this current equals the potential across battery I2 and it is applied to condenser C through timing resistor R. Condenser C begins to charge, and the potential across the condenser increases exponentially as governed by the time-constant of serially connected resistor R and condenser C. Considering the potential between point P and ground, battery I2 supplies a xed potential of negative polarity which is partially counteracted by the potential across condenser C. The sum of these two potentials at any instant equals the potential between point P and ground. The potential across the condenser C rises exponentially, as governed by the characteristics of the time-delay network comprising resistor R and condenser C, until the potential from point P to groundis less than the potential applied to the right side of the unilateral cell by battery I3.

When the potential from point P to ground just equals the potential of battery I3, there is no bias across cell I5 since there is no potential difference between the right side and the left Vside of the cell. Therefore unilateral cell I5 becomes `conducting and the resistance which it oiers to the ow of current from battery I3 is negligible. Current now. flows to the negative terminal of battery I3 through the winding of relay I4, cell I5, resistor R, and switch` I from the positive terminal ofbattery I3.- The current through this branch of the circuit increases rapidly lto its steady state value, and the rate of increase is governed by the impedance of the series circuit comprising the winding of relay I4, cell I5, and resistor R. When the current has increased to the magnitude required to operate the relay (designated Operate Current in the drawing), the armature vof relay I4 operates. As soon as cell I becomes conducting both batteries I2 and I3 cause current to pass to switch IIJ from ground and the potential then applied to condenser C equals the potential drop across resistor II less the potential drop across resistor R caused by the sum of the charging current supplied by battery I2 and the current owing from battery I3. The potential drop caused by the sudden current which flows from battery I3 through resistor R reduces the potential applied to condenser C since the potential drop across resistor R is of opposite polarity to the potential applied to the condenser across resistor II. The potential nowv applied to condenser C is less than the potential applied before cell I5 became conducting, therefore, the rate of charging condenser C is decreased and the charging current decreases rapidly to zero in an exponential manner. Typical curves showing the current and voltage relationships after contacts III are closed are indicated in Fig. 2 and Fig. 3.

From an inspection of the curves in Fig. 2 andl Fig. 3 it may be seen that the ability of the circuit to cause an abrupt rise in current in the relay branch, following a precisely timed interval, makes variations in relay operate time'a relatively small partof the over-all time thereby permitting precise control over the time-delay inter val. When contacts III are opened condenser C discharges through resistors R and and II and unilateral cell I5 is again biased so that no current is permitted to flow through the relay branch of the circuit. The time required for discharging condenser C and thereby restoring the bias to the unilateral cell I5 can be reduced by shunting the timing resistor R with an additional unilateral cell poled in the opposite direction to cell I5.

The time which elapses between the operation of switch III and the operation of the armature of relay I4 is determined by the time-constant Iof serially connected resistor R and condenser C, by the difference in potential between batteries I2 and I3, and by the impedance of the series circuit comprising the winding of relay I4, cell I5, and resistor R. 'Ihe time-constant of serially connected resistor R and condenser C and the difference in potential between batteries I2 and I3 determine the time at which cell I5 becomes conducting. The impedance of the series circuit comprising the winding of relay I4, cell I5, and resistor R determines the rate at which the current through the winding of relay I4 increases after cell I5 becomes conducting. It will be observed that variations in time between the instant 4 relay winding is energized and the instant the relay armatuteis operated can be made a relatively small part of the over-al1 time delay so that precise control over the operation of the relay may be obtained.

The electrical values of the time-delay network are determined by the potential difference .between batteries I2 and I3, the potential of battery I 2. and the time-delay desired. During the time interval between ,the operation of switch I0 and me time when cell I5 becomes conducting the potential across condenser C is determined by the following equation:

` base of natural logarithms, t is the elapsed time,

R is the res `tance of resistor R, and C is the capacitance o condenser C. Cell I5 becomes conducting when where V13 is the potential across battery I3.

The value of resistance II is not critical. II the resistance is large, the current drain on battery I2 is small but the time required to discharge condenser C and restore the circuit is large. If the resistance is small, the current drain on battery I 2 is large and the time required to discharge condenser C and restore the circuit is-small.

As pointed out above, the impedance of the series circuit comprising the winding of relay I4. cell I5, and resistor R determines the interval of time between the instant cell I5 becomes conducting and the instant the relay armature operates. The inductance of the winding of relay I4 is usually the only important element which must be considered. It is desirable to minimize the interval of time between the instant that cell I5 becomes conducting and the instant the relay armature operates so as to minimize any variations in the time-delay which may be introduced during that portion of the total time. Thus, the inductance of the winding of relay I4 should be minimized in order to obtain precise control over the timing action.

Fig. 4 indicates a circuit using a single source of potential 4I and a voltage divider made up of resistors 42 and 43 to achieve the same result as the basic circuit. The operation of this circuit differs from that of the basic circuit in that when contacts 40 are open there is no biasing potential applied to unilateral cell 45; however, the instant contacts 40 are closed bias is applied to unilateral cell 45 from the voltage divider comprising resistors 42 and 43, and the circuit then functions in the same manner as the basic circuit. The circuit is restored by opening contacts 40 thereby opening the potential circuit and permitting timing condenser C to discharge through timing resistor R and the two elements 42 and 43 of the voltage divider.

Fig. 5 shows the circuit indicated in Fig. 4 modied so that the connection between resistors 53 and R is made through contacts 55 which are an armature and back contact of relay 54. When bias is removed from unilateral cell 55 by the build-up of sufficient potential across condenser C a, sudden current is permitted to flow through the winding of relay 54 as described for the basic circuit. When relay 54 operates and opens contacts 56 the circuit branch comprising resistor timing condenser and against the front contacts of relay 54. In par-l ticular, after capacitor C has charged sufliciently to cause relay 54 to operate, the potential of the relay terminal connected to the junction of resistors 52 and 53 assumes an abrupt change of K potential in the negative direction by the opening of contact inasmuch as resistor 53 oats potentialwise, and in view of the fact that the other terminal of winding 54 ismaintained at substantially the same potential with respect to ground, the opening of contact 56 impresses an increased potential across winding 54 thereby immediately accelerating the armature motion. When contacts 50 are opened the flow of current from battery 5| is interrupted, the armature of relay 54 is restored to the back contacts due to the action of the armature spring, and contacts 56 are closed. The closure of contacts 56 establishes a discharge path for condenser C compris*-` ing resistors 52, 53 and R.

Although it is 'apparent that the circuit parameters will be selected in accordance with the available voltage sources and the desired time delay, the circuit of Fig. 1, for example, has been demonstrated properly to operate with the followingA parameters: Resistor R, 10,000 ohms; resistor l I, 500 ohms; condenser C, 12 microfarads; battery l2, negative 50 volts; battery I3. negative 20 volts; winding of relay I4, 8,450 ohms.

As a further example, the circuit of Fig. 4 has been demonstrated properly to operate with the following parameters: Resistor R, 10,000 ohms; resistors 42 and43, 250 ohms; condenser C, 12 microfarads; battery 4|, negative 50 volts; winding of the relay, 8,450 ohms.

,What is claimed is:

1. A time-delay circuit comprising a voltage divider, a serially connected circuit comprising a unilateral cell, relay, rst element of said voltage divider, and timing condenser, a second element of ,said voltage divider connected to the junction between said first element and said relay. a timing resistor connected in series with said second element of the voltage divider and connected to the junction between said timing condenser and said unilateral `cell, a source of potential connected to ground and to the junction between the the ilrst element of the voltage divider, and a set of contacts connected to ground and the junction between the second element of the voltage divider and the timing resistor whereby bias is applied to the unilateral cell from the voltage divider the instant said contacts are closed, and, due to the action of the timing resistor and condenser, said bias is removed a timed interval thereafter thereby permitting a suddeny current to pass through the unilateral cell and operate said relay.

2. A time-delay circuit comprising a voltage divider, a serially connected circuit comprising a unilateral cell, relay,

nected timing resistor and set of contacts circuit connected to ground and to the junction between the unilateral cell and timing condenser, a second element of said\voltage divider connected to the junction between said iirst element and said relay, a back contact and armature of said relay connected in series with said second element of the voltage divider and connected to the junction between the timing resistor and the set of contacts which are connected to ground, and a source of potential connected to ground and to the junction between the timing condenser and the rst element of the voltage divider whereby bias is applied through the timing resistor to the unilateral cell the instant said serially connected timing resistor and set of contacts circuit is closed, and, due to the action of the timing condenser and resistor, said bias is removed a timed interval thereafter thereby permitting a sudden current to pass through 'the unilateral cell and operate said relay, the operation of said relay serving to open said back contact and armature thereby suddenly increasing the voltage applied to said relay and accelerating the movement of the relay armature.

FRAIK K. LOW.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED sTATs PATENTS Number Great Britain May 29, 1947 first Velement of said volt- 'age divider, and timing condenser, a serially con- 

